Embodiments of the present invention are directed to power supply control circuits and power supply systems. More particularly, embodiments of the invention provide methods and circuits for constant-voltage and constant-current control of non-isolated buck converter switched mode power supply (SMPS). But it would be recognized that the invention has a much broader range of applicability.
Regulated power supplies are indispensable in modern electronics. For example, the power supply in a personal computer often needs to receive power input from various outlets. Desktop and laptop computers often have regulated power supplies on the motherboard to supply power to the CPU, memories, and periphery circuitry. Regulated power supplies are also used in a wide variety of applications, such as home appliances, automobiles, and portable chargers for mobile electronic devices, etc.
In general, a power supply can be regulated using a linear regulator or a switching mode controller. A linear regulator maintains the desired output voltage by dissipating excess power. In contrast, a switching mode controller rapidly switches a power transistor on and off with a variable duty cycle or variable frequency and provides an average output that is the desired output voltage.
Compared with linear regulators, switching mode power supplies have the advantages of smaller size, higher efficiency and larger output power capability. Pulse Width Modulation (PWM) and Pulse Frequency Modulation (PFM) are two control architectures of switching mode power supplies. Under PWM (pulse width modulation) control, the switching frequency of the power switch is fixed value, but the turn-on time of the power switch is regulated according to the sampled output voltage to ensure that the energy in the inductor is maintained at a constant in order to provide a constant output voltage. Under PFM (Pulse Frequency Modulation) control mode, the control module, based on the sampled value of the output voltage, controls the power switch off-time after the end of the inductor discharge to regulate the next turn-on time of the power switch. In PFM, the on time of the power switch does not vary with the change in load condition.
Switched mode power supply systems can be isolated or non-isolated. In isolated power supplies, a transformer is used to transfer energy between the primary side and the secondary side. In a non-isolated switched mode power supply system, a power switch is connected to an inductor, which is also connected to the output. Magnetic energy is stored in the inductance when the switch is turned on, and the energy is transferred to the output when the switch is turned off. A non-isolated buck converter has the advantage of simple topology and lower cost. But non-isolated converters are often limited in either constant voltage or constant current control, and it is difficult to provide both constant voltage (CV) and constant current (CC) control.
Therefore, there is a need for techniques that can provide both constant voltage (CV) and constant current (CC) control for non-isolated buck converters.